The glucose content in blood is considered an important marker for diabetes. A diagnosis of diabetes is made by a simplified measurement (Point-of-Care Testing: POCT) such as a simplified test conducted by clinical staff or the like, or a self-inspection conducted by a patient, in addition to a clinical examination conducted in a hospital examination room or the like.
Although the simplified measurement is conducted using a glucose diagnostic kit or a measurement apparatus such as a biosensor or the like (POCT apparatus), a glucose oxidase is conventionally used in the POCT apparatus. However, the glucose oxidase depends on dissolved oxygen concentration, and thereby errors occur in measured values. Accordingly, use of glucose dehydrogenase which is not influenced by oxygen is recommended.
There are, as the glucose dehydrogenase, NAD coenzyme-unlinked glucose dehydrogenases of which the coenzyme is nicotinamide adenine dinucleotide (NAD) or nicotinamide adenine dinucleotide phosphate (NADP) and coenzyme-linked glucose dehydrogenases of which the coenzyme is pyrroloquinoline quinone (PQQ), flavin adenine dinucleotide (FAD), or the like. Among them, the coenzyme-linked glucose dehydrogenases are advantageous in that they are less liable to be affected by contamination components in comparison with the NAD coenzyme-unlinked glucose dehydrogenases, and they realize high measurement sensitivity and production of the POCT apparatuses at low cost.
However, conventional pyrroloquinoline quinone (PQQ)-type glucose dehydrogenases are disadvantageous in that the stability thereof is low, and they easily react with maltose or galactose. Maltose is a sugar used in a transfusion. When the PQQ-type glucose dehydrogenases react with maltose, the POCT apparatus which measures blood sugar levels indicates a higher blood sugar level than an actual blood sugar level. As a result, the patient injects an excessive amount of insulin, and thereby suffers from hypoglycaemia, which causes consciousness disorder or a comatose state, which has attracted tremendous interest.
In particular, the blood sugar POCT apparatus is used to measure the blood sugar level and the importance thereof has increased due to its convenience in patient self-care and medication, and thus self blood sugar monitoring apparatuses (Self-Monitoring of Blood Glucose: SMB) have been increasingly used in the home. Accordingly, the demand for realizing measurement accuracy is deemed to be very high.
In actuality, a notification calling for attention with respect to use of blood sugar testing apparatuses with an enzyme that utilizes PQQ as a coenzyme was issued to patients receiving maltose transfusion from the Japanese Ministry of Health, Labor and Welfare on February, 2005 (Feb. 7, 2005; Pharmaceutical and Food Safety Notification No. 0207005, and the like).
On the other hand, there have been reported, as the coenzyme-linked glucose dehydrogenases which catalyze dehydrogenation of glucose with FAD as the coenzyme, enzymes originating from Agrobacterium tumefaciens (J. Biol. Chem. (1967) 242: 3665-3672), enzymes originating from Cytophaga marinoflava (Appl. Biochem. Biotechnol. (1996) 56: 301-310), enzymes originating from Halomonas sp. α-15 (Enzyme Microb. Technol. (1998) 22: 269-274), enzymes originating from Agaricus bisporus (Arch. Microbiol. (1997) 167: 119-125, Appl. Microbiol. Biotechnol. (1999) 51: 58-64), and enzymes originating from Macrolepiota rhacodes (Arch. Microbiol. (2001) 176: 178-186). These enzymes oxidize a hydroxyl group at the 2-position and/or 3-position of glucose, and exhibit a high activity toward maltose, but low selectivity to glucose. Although coenzyme-linked glucose dehydrogenases originating from Burkhorderia cepacia with a high activity toward maltose are also known, their natural type enzyme is a heterooligomer enzyme composed of three subunits α, β, and γ, and known as a membrane-binding enzyme. Accordingly, there are disadvantages in that solubilization treatment is required to obtain the enzyme, and cloning of necessary subunits is simultaneously required to realize sufficient activity by cloning.
In the Society for Biotechnology, Japan (October 28 to 30, 2002), there was a presentation regarding the substrate selectivity (activity against maltose and activity against galactose, with respect to the activity against glucose which is assumed to be 100%) in which SM4 strain exhibited 40% and 105%, JCM5506 strain exhibited 43% and 132%, JCM550 strain exhibited 57% and 123%, JCM2800 strain exhibited 83% and 108%, JCM2801 strain exhibited 74% and 117%, IFO14595 strain exhibited 38% and 104%, and IFO15124 strain exhibited 74% and 148%, and the presenter thereof stated that these strains exhibited high activity against maltose, which was disadvantageous if used for a self blood sugar monitoring apparatus, and therefore the presenter was going to improve the substrate selectivity by changing the sequence thereof.
In contrast, inventors of the present invention invented a novel soluble coenzyme-linked glucose dehydrogenase of which the coenzyme is FAD and which is not a membrane-bound type, and filed a patent application (Patent Document 1). The coenzyme linked glucose dehydrogenase disclosed in Patent Document 1 oxidizes a hydroxyl group at the 1-position of glucose, is excellent in substrate-recognizing ability against glucose, is not influenced by dissolved oxygen, and exhibits low activity toward maltose (activity against maltose of 5% or less and activity against galactose of 5% or less, with respect to the activity against glucose which is assumed to be 100%), such excellent characteristics not being realized by conventional ones.
However, the coenzyme-linked glucose dehydrogenase disclosed in Patent Document 1 is isolated or extracted from liquid culture medium in which wild microorganisms (such as, for example, microorganisms belonging to the genus Aspergillus) are cultivated, and therefore, the production yield thereof is limited. In addition to the slight production yield of the enzyme, large amounts of sugar are bound to the enzyme, and therefore the enzyme is in a so-called “sugar-embedded-type enzyme” from which is coated by different kinds of sugar from N-type or O-type sugar chain which binds to general enzymes, as a result of which the activity thereof is difficult to be detected (that is, the enzyme activity is low), the sugar chain cannot be removed enzymatically or chemically, and thereby, the enzyme is scarcely stained by usual protein staining (using Coomassie Brilliant Blue G-250 or the like) after electrophoresis, and a terminal or internal amino acid sequence of the enzyme, which is necessary information for obtaining the gene, is difficult to decode by performing conventional purification. Accordingly, there is no case in which cloning of the enzyme gene succeeds to ascertain expression of the enzyme activity.
Although the existence of coenzyme-linked glucose dehydrogenases originating from Aspergillus oryzae was suggested in 1967 (Non-patent Document 1), only partial enzymatic properties thereof were revealed. Although the dehydrogenase was suggested to provide no influence on maltose, there are no detailed reports regarding the coenzyme-linked glucose dehydrogenases originating from Aspergillus oryzae, and no reports regarding coenzyme-linked glucose dehydrogenases originating from other microorganisms which oxidize a hydroxyl group at the 1-position of glucose, and also no reports regarding amino acid sequences or genes of the coenzyme-linked glucose dehydrogenases are known.
Although an idea of measuring glucose using a glucose dehydrogenase EC 1.1.99.10 is known (see Patent Document 15), there is no case in which any coenzyme-linked glucose dehydrogenases are produced to a practical level, and therefore, no coenzyme-linked glucose dehydrogenases have been developed for practical use in a sensor. The reason for this is the activity of the enzyme in the fungus body is weak, and even if the enzyme is secreted outside the fungus body, the amount thereof is extremely slight, and the activity thereof is weak because the enzyme is coated by a large amount of sugar, as a result of which the enzyme is difficult to detect. Accordingly, it is speculated that a gene of the enzyme can not be cloned.
It is been known that the measurement of glucose levels using a sensor utilizing a glucose oxidase is influenced by sugar chains of the enzyme, and thereby it is difficult for an enzyme originating from molds rich in sugar chains to be adapted to the glucose sensor (Non-patent Document 2). It is known, for example, that solid cultivation of microorganisms belonging to the genus Aspergillus increases the sugar content of yielded enzymes in comparison with liquid cultivation thereof (Non-patent Document 3), and thus it is known that solid cultivation generally increases sugar chains in comparison with liquid cultivation. Thus, one of reasons coenzyme-linked glucose dehydrogenases have not been developed for practical use until now is assumed to be because it has been difficult to reduce sugar chain contents of the glucose dehydrogenases originating from molds to utilize it in a glucose sensor even if cultivating conditions are investigated.
In fact, although the present inventors purified a coenzyme-linked glucose dehydrogenase originating from Aspergillus terreus, the inventors found that the obtained dehydrogenase was coated with a great amount of sugars to be in a form which may be called an “arabinogalactan embedded-type enzyme”, as a result of which an enzyme-immobilized electrode formed by applying the enzyme on an electrode and then drying is not sufficiently dried, and the reactivity of a glucose sensor is deteriorated by the existence of the sugars.
Biogenetic methods in which gene stocks encoding proteins such as enzymes or the like are utilized to produce the proteins on a massive scale are known, and biogenetic methods for preparing glucose dehydrogenases as disclosed in Patent Documents 2 to 14 are known. These mainly relate to modification of PQQ glucose dehydrogenases, and provide modified PQQ glucose dehydrogenases, in which disadvantages of conventional PQQ glucose dehydrogenases, such as low substrate selectivity and low stability, are improved, and modified gene stocks for biogenetically preparing the modified PQQ glucose dehydrogenases.
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